It is well known that three-phase generators cause operating noise which arises due to changes in the reluctance of the magnetic circuit as the rotor operates, in the region of the transition between stator to rotor. Looked at from another point of view, the permeability--which is the reciprocal of reluctance--of the path which includes the stator and the rotating rotor changes. It has previously been proposed to introduce ferromagnetic slot closers within the slots formed in the stator. The stator slots usually are radially directed slots in which the stator windings are introduced. Ferromagnetic slot closers have been used to reduce the magnetic reluctance, and thus reduce the effect of changing forces, and their amplitudes, which arise between the stator and the rotor in operation of the machine. Placing ferromagnetic slot closers in all the slots of a stator thus permits some extent of reduction of noise; its has been found, however, that the overall reduction which can be effected thereby is not sufficient for low-noise operation of the machine, due to noise which still arises based on changes in the reluctance or permeability of the magnetic circuit.